Physiological studies have established that auditory neurons can be classified into two categories based upon their spontaneous rate (SR): low-SR neurons have higher thresholds and wider dynamic ranges than high-SR neurons. Our hypothesis is that high-SR neurons are primarily responsible for detection at threshold level whereas low-SR neurons are primarily responsible for suprathreshold discrimination tasks. The long-term objectives are to determine the contributions of high- and low-SR neurons to psychophysical performance in normal-hearing listeners and to differentiate possible functional deficits that hearing-impaired listeners may have due to the selective damage of the two groups of SR neurons. The specific aims are to assess the differential contributions of the two types of SR neurons to (1) intensity discrimination; (2) loudness sensation; (3) frequency discrimination; (4) pitch sensation; and (5) stop-consonant discrimination. The experimental design is based upon Relkin and Doucet's (1991) finding that low-SR neurons have a much slower recovery (2000 msec) from forward masking than high-SR neurons (100-200 msec). At signal delays of 100-200 msec, high-SR neurons are recovered from forward making while low-SR neurons are not. Thus, psychophysical measurements at a signal delay of 100-200 msec following a forward masker can differentiate the contribution of high- and low-SR neurons. The methods to be used are two- interval forced-choice adaptive procedure in experiments described in specific aims (1)-(4), and single-interval constant stimuli procedure in stop-consonant discrimination experiment (specific aim 5). Testing hypothesis put forth in this proposal could lead to better understanding the treatment of the speech recognition deficits in hearing- impaired listeners.